Mount sdcard USB Storage through adb.

I can't seem to find a command that will allow me to mount/unmount the
sdcard from the adb prompt.

From the documentation, it appears that this can't be done in an
emulator, so I am trying with a Nexus One.

I would like to simulate the card being mounted or unmounted while my
Activity is active. I can't do this on the Nexus One through the
notification menu, because this will pause and then resume and
activity, during which time storage card broadcast notifications are
not sent.

I know other devices can and do mount the storage without leaving an
Activity, in particular because they will automount the storage
whenever the USB is plugged in (according to users, even if you tell
it not to). For that reason, this is a case I want to test.